1. Technical Field
The exemplary features presented relate to a photovoltaic power harvesting system including multiple photovoltaic strings and, more particularly to system and method for maximizing power in each photovoltaic string.
2. Description of Related Art
Reference is made to FIG. 1 which shows a photovoltaic power harvesting system 10 according to conventional art. A photovoltaic string 109 includes a series connection of photovoltaic panels 101. Photovoltaic strings 109 may be connected in parallel to give a parallel direct current (DC) power output. The parallel DC power output connects to the input of a direct current (DC) to alternating current (AC) inverter 103. The AC power output of inverter 103 connects across an AC load 105. AC load 105 may be an AC load such as an AC motor or may be an electrical power grid.
By way of a simplified numerical example, three strings 109 may be used with an inverter 103. If two strings 109 are equally irradiated such that each string operates with a string voltage of 600 volts (V) and string current of 10 amperes (A); each of the two strings generates (10 A·600 V) 6 kilowatts (kW). It is also assumed that the two equally irradiated strings 109 may be operating at maximum power.
If however, one string 109 is partially shaded or if one or more panels 101 is under performing, there may still be a string voltage of 600V as set by the other two equally irradiated strings 109, however, the string current in the one under performing string 109 may only be only 6 amperes. The under performing string 109 is not operating at maximum power point. For instance, it may be that the under performing string 109 has a maximum power point of 550 volts for a current of 10 amperes. In this situation, the power lost by the under performing string 109 is 1.9 kW (550V·10 A−600V·6 A). The under performing string 109, therefore, produces 3.6 kW (600V·6 A). Overall power harvested from system 10 is, therefore 15.6 kW (3.6 kW+2·6 kW).
Reference is now made to FIG. 2 which shows another power harvesting system 20 according to conventional art, according to international patent application publication WO2010002960. System 20 is directed to reduce power losses compared to the losses of system 10. Each photovoltaic string 109 includes a series connection of photovoltaic panels 101. Each photovoltaic string 109 is connected in parallel to an input of a DC-to-DC converter 205. The output of converter 205 connects to a DC bus 211. The DC voltage generated by photovoltaic string 109 is converted by converter 205 to the voltage of DC bus 211. Each photovoltaic string 109 together with the respective DC-DC converter 205 forms a photovoltaic string module 207. A number of modules 207 with outputs from respective DC-to-DC converters 205 may be connected in parallel to DC bus 211. The parallel combined outputs of modules 207 may be also connected to an input of a direct current (DC) to alternating current (AC) inverter 103 via DC bus 211. Inverter 103 converts the combined DC power outputs of modules 207 to an alternating current power at an output of inverter 103. The output of inverter 103 connects to AC load 105.
Still referring to FIG. 2, using the same numerical example as in system 10 (FIG. 1), three modules 207 may be used with inverter 103. Two strings 109 may be equally irradiated such that each string of the two strings operates with a string voltage of 600 volts and string current of 10 amperes. Each of the two strings generates (10 amperes·600 volts) or 6 kilowatts. If the one remaining string 109 is under performing, there may be maximum power point for the under performing string 109 of 550 volts and current of 10 amperes. Each DC-to-DC converter 205 may be configured to maximize power on each respective output to give 600 volts on DC bus 211. The two equally irradiated modules 207 each produce 6 kW (10 amperes·600 volts) and the under performing unit 207 produces 5.5 kW (10 amperes·550 volts). Giving an overall power harvested from system 20 of 17.5 kW. It can be seen that system 20 offers an improvement of 1.9 kW over system 10 in terms of minimized losses and increased power harvested. The improvement has been achieved through multiple DC-DC converters 205 which operate at wattage levels of around 6 kW. The high power DC-DC converters 205 in a power harvesting system may add to the cost of installation and maintenance of the power harvesting system and may present an overall decreased level of reliability for the power harvesting system because DC-DC converters 205 operate at high wattage levels.
The terms “monitoring”, “sensing” and “measuring” are used herein interchangeably.
The terms “power grid” and “mains grid” are used herein interchangeably and refer to a source of alternating current (AC) power provided by a power supply company.
The term “converter” as used herein applies to DC-to-DC converters, AC-to-DC converters, DC-to-AC inverters, buck converters, boost converters, buck-boost converters, full-bridge converters and half-bridge converters or any other circuit for electrical power conversion/inversion known in the art.
The term “DC load” as used herein applies to the DC inputs of converters, batteries, DC motors or DC generators.
The term “AC load” as used herein applies to the AC inputs of converters, transformers, AC motors or AC generators.